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This talk analyzes the limits that quantum mechanics imposes on the
accuracy to which spacetime geometry can be measured. By applying the
fundamental physical bounds to measurement accuracy ensembles of clocks
and signals, as in the global positioning system, I present a covariant
version of the quantum geometric limit, which states that the total
number of ticks of clocks and clicks of detectors that can be contained
in a four volume of spacetime of radius R and temporal extent is less
than or equal to RT divided by the Planck length times the Planck time.
The quantum geometric bound limits the number of events or `ops' that
can take place in a four-volume of spacetime and is consistent with and
complementary to the holographic bound which limits the number of bits
that can exist within a three-volume of spacetime.